Shirotaka didn't explicitly tell him that it started out as just for fun to tease Fujiwara until ch 17. At this point, Fujiwara's antennae are up because Hitsuji blurted her being-played role the jealousy plan. He continues to be close and sweet to her, and then when she responds to it, he goes cold because of his own rules, and agrees that he is in no position to say anything about whatever relationship her and Shirotaka have. None of that (her feelings about his warm/cold treatment , Fujiwara's struggle with his own feelings, his friends' supposed relationship) stops him from hugging his friend's girlfriend. Even if he thinks something is up, it doesn't seem like he's doing any of this to guard her from, say, being played his friend, it just seems like he's doing whatever he feels like doing.